fix(ci): add C++20 compiler flag for better-sqlite3 builds

Without this patch, the GitHub Actions workflow fails when building
better-sqlite3 native modules because node-gyp attempts to compile
them with the default C++ standard, which is insufficient for Node.js
24's requirements.

This is a problem because better-sqlite3 (required by the project)
needs C++20 or later to compile successfully, causing all CI builds
to fail with "C++20 or later required" compilation errors.

This patch solves the problem by adding the CXXFLAGS environment
variable with '-std=c++20' to all pnpm install steps in the build,
test, and verify jobs. This ensures node-gyp uses C++20 when
compiling native addons.

Co-authored-by: Claude <noreply@anthropic.com>

Author: Adam Spiers

.github/workflows/repo.yaml

@@ -34,6 +34,8 @@ jobs:
           path: ~/.cache
           key: ${{ env.CURRENT_MONTH }}-${{ runner.os }}
       - run: pnpm i --frozen-lockfile
+        env:
+          CXXFLAGS: '-std=c++20'
       - run: pnpm build
       - uses: actions/upload-artifact@v4
         with:
@@ -71,6 +73,8 @@ jobs:
           path: ~/.cache
           key: ${{ env.CURRENT_MONTH }}-${{ runner.os }}
       - run: pnpm i --frozen-lockfile
+        env:
+          CXXFLAGS: '-std=c++20'
       - uses: actions/download-artifact@v4
         with:
           name: dist
@@ -101,6 +105,8 @@ jobs:
           path: ~/.cache
           key: ${{ env.CURRENT_MONTH }}-${{ runner.os }}
       - run: pnpm i --frozen-lockfile
+        env:
+          CXXFLAGS: '-std=c++20'
       - uses: actions/download-artifact@v4
         with:
           name: dist